We design and demonstrate a cascaded SiN-based RGB coupler with dual-mode interference (DMI) for a micro laser scanning image projector. The DMI configuration in SiN-based waveguides mitigates the adverse effects of self-image point deviation caused by wavelength dispersion, achieving decreased device length and high transmission efficiency. The underlying mechanisms are discussed based on coupled-mode theory and 3D finite difference time domain simulation. A small footprint of 3 µ m × 70 µ m is achieved for the RGB coupler device without input/output circuits, and the insertion losses are less than 0.56 dB at RGB wavelengths. There are two orders of magnitude reduction in device length as compared with the conventional S i O 2 -based RGB coupler, which greatly promotes the miniaturization of the couplers and displays integration advantages with a laser diode and waveguide photodetector. In addition, the 3 dB bandwidth is over 50 nm for the coupler, and it demonstrates good fabrication tolerance. This design can further be integrated into visible light communication systems and applied to visible light integrated photonics.
We present an approach to the design of an RGB coupler for miniature projection and near-eye display devices using a photonic crystal-like sub-wavelength (PhC-SW) structure based on the SiN-SiO2 photonic integration platform. The proposed device was optimized using an inverse design method. By etching pixelated air holes on the SiN core layer, light propagation can be controlled by modulating the equivalent refractive index. This approach results in a device with an ultra-compact size of 3×4µm 2 , which is one of the smallest reported design size for the RGB coupler. In addition, the proposed design could achieve transmission efficiencies of 72.10%, 84.41% and 90.16% insertion losses (IL) of -1.42dB, -0.74dB and -0.45dB, and 3dB bandwidth of -270.73dB, -248.20dB and -224.32dB at the RGB lights, respectively. This new method would provide a compact and efficient solution for the miniaturization of RGB couplers.
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